The invention relates to apparatus and methods for converting a frequency division multiplexed (fdm) signal into a time division multiplexed (tdm) signal. It finds particular but not exclusive application in a communication system comprising a primary station and a plurality of secondary stations, each of the stations having signal transmitting means and signal receiving means, a first communication link extending from the primary station to signal splitting means for supplying signals from the primary station to each of the secondary stations, and a second communication link extending from signal combining means, which receives signals from each of the secondary stations, to the primary station.
Such communication systems find application in a variety of fields including local telephone networks and the like. One method of operating such a communication system is to distribute signals from the primary station (or exchange) in multiplexed form, for example a common time division multiplexed signal, so all the secondary stations. Each station them selects the time slots appropriate to it. Although this is relatively straight forward, difficulties arise if the secondary stations are to transmit information back to the primary station by this method. In this case, the signals transmitted from the secondary stations must be accurately timed to avoid signal "collisions" in the common path back to the primary station. The timing will depend principally on the path length and, in the case of optical communication systems using optical fibres, this path length can vary slightly with temperature effects. Thus a ranging system must be built into each terminal.
These difficulties can be obviated by combining the return signals from the secondary stations into a frequency division multiplexed signal instead of attempting to combine these signals into a time division multiplexed signal. This avoids the problems of timing due to variations in temperature and the like.
Typically, the signals transmitted between the stations will comprise optical signals and in this case the stations may be coupled by optical waveguides such as optical fibres. However other media, including air, could define the communication links. Furthermore, the signals could comprise electrical or radio frequency signals.
In some cases, separate paths may be provided between the primary station and the secondary stations to carry signals in respective directions. Conveniently, however, a common transmission path is used, for example a common optical waveguide in the case of optical signals. In the latter case the signal combining and splitting means are provided by a common element. In the case of a common transmission path, the signals could be transmitted with the same or different wavelengths in the opposite directions.
The primary station receiving means may include a splitting means to split the incoming frequency division multiplexed signal into a plurality of subsidiary signals, one for each secondary station, and a plurality of demodulating circuits for receiving respective ones of the subsidiary signals and for regenerating the information associated with the signal from the corresponding secondary station. However, this arrangement needs separate demodulating circuits for each channel or secondary station.